An image forming apparatus is configured such that a photosensitive body and the light-emitting face of an exposing unit (for example, a light-emitting diode (LED) head or the like) are disposed close to each other and the photosensitive body is exposed. Such image forming apparatus is required to have a mechanism that keeps the distance between the photosensitive body and the exposing unit constant. For example, in the image forming apparatus described in JP-A-1-279272, this distance is kept constant by providing a guide roller to an LED head to contact with a photosensitive drum.
The direction in which the guide roll rotates as the photosensitive body is moved, that is, the direction of the relative movement of the guide roller with respect to the photosensitive body, is determined by the posture of the guide roller. However, even if the rotation direction of the guide roller, that is, the posture of the guide roller, is adjusted so as to be accurately aligned with the movement direction of the photosensitive body, the guide roller may tend to rotate obliquely with respect to the movement direction of the photosensitive body due to an adjustment error and the clearance between the guide roller and the supporting shaft thereof. This oblique rotation of the guide roller with respect to the movement direction of the photosensitive body causes the LED head to wobble about the photosensitive body and cause deterioration in image formation.